1. Field of the Invention
The present invention relates to a hydraulic control system of an automatic transmission.
2. Related Art
Generally, an automatic transmission for an automotive vehicle is provided with a torque converter and a shift speed mechanism. The torque converter converts a torque produced by a crank shaft or an engine torque and transmits an engine output torque to a turbine shaft as a turbine torque. The shift speed mechanism mechanically converts the turbine torque and introduces an output shaft. Meanwhile, the shift speed mechanism is, generally, provided with a planetary gear mechanism having a plurality of gears such as a sun gear, ring gear, pinion gear and the like and employed to switch a power transmitting path in its shift operation and thus switches shift characteristics (shift stage, shift ratio).
In order to change the power transmitting path or to change the shift characteristics of the shift speed mechanism, the automatic transmission is provided with a number of hydraulic frictional elements, such as clutches for switching on a gear (transmission of the torque) or off a gear(interruption of the transmission of the torque ), brakes for braking on a gear (engage) or off (release) and the like. The hydraulic mechanism switches the on-off pattern of the respective frictional elements to thereby switch the shift characteristics of the shift speed mechanism.
In such an automatic transmission, an engaging force of the frictional element is substantially proportional to a line pressure or basic supply pressure. In a shift operation, the engaging pressure of the frictional elements, in other words, the line pressure of the hydraulic mechanism must have an appropriate value in accordance with a torque amount transmitted through the frictional element. If the unduly high value of the line pressure incurs a power loss as well as an undesirable shift shock. Conversely, the unduly low line pressure extends a time period for the shift operation resulting in a deterioration of the running performance of the vehicle. The line pressure must have an appropriate value at a normal condition other than the shift condition as well in accordance with the torque amount transmitted through the shift speed mechanism. As mentioned above, if the line pressure is too high, the power loss is increased. And, if the line pressure is too low, the frictional element produces a slip therebetween so that an engaging portion of the frictional element is undesirably worn and heated.
In view of this, the conventional automatic transmission usually controls the line pressure in accordance with the turbine torque which is an input torque from the torque converter to the shift speed mechanism. However, it is difficult to directly detect the turbine torque or a torque of the turbine shaft. Therefore, conventionally, an engine output torque or crank shaft torque is computed based on the ignition timing and the like, then engine torque is multiplied by the torque ratio of the torque converter to compute the turbine torque. Meanwhile, the torque ratio of the torque converter can be easily calculated by a ratio of the turbine shaft rotation speed to the crank shaft rotation speed or a speed ratio.
The turbine shaft speed, in other words, the input member speed of the shift speed mechanism changes during the shift operation. Therefore, a moment force derivable from a moment of inertia of the power transmitting system from the engine to the shift speed mechanism acts on the frictional element due to the change of the input member speed. Thus, the torque amount transmitted through the frictional element is a sum of the input torque of the shift speed mechanism and the moment force of the power transmitting system. Specifically, in case of an up shift operation, the input member speed is reduced so that the moment force acts on the frictional element in the same direction as the turbine shaft torque. Conversely, in case of a down shift operation, the input member speed is increased to that the moment force acts on the opposite direction to the turbine shaft torque.
Meanwhile, the moment force is a product of the moment of inertia and an angular acceleration of a shaft.
Thus, the frictional element is subjected to the moment force as well as the turbine torque so that the optimized line pressure cannot be determined in the shift condition based solely on the input torque introduced into the shift speed mechanism. In view of this, it has been proposed that a hydraulic control device determines the line pressure based on the input torque to the shift gear mechanism and the input speed thereto (see for example Japanese patent publication 4-72099 published in 1992 for opposition).
It should however be noted that the conventional hydraulic control system for the automatic transmission as disclosed in the Japanese patent publication No. 4-72099 in which the line pressure is determined based on the input torque and input speed to the shift gear system is disadvantageous in that the system cannot estimate accurately the moment force acting on the frictional element during the shift condition. In the up-shift operation, the output torque of the shift speed mechanism is temporarily increased due to the moment force so that a torque shock is produced. In order to suppress the increase the output torque, the shift operation time period is necessary to be extended so that the running performance during the shift operation is deteriorated.